The long-term goal of this project is to explore the molecular mechanisms underlying cellular sensitivity to ionizing radiation (IR) and to devise strategies to regulate radiosensitivity by manipulating signal transduction events. Signal transduction events, especially those mediated by the oncogenic Ras and epidermal growth factor receptor (EGFR), can significantly alter the radiosensitivity of tumor cells and cause them to become more resistant to ionizing radiation-induced cell killing. However, the precise mechanisms underlying the actions of EGFR and Ras are not well understood. This application proposes to adopt a multidisciplinary (biochemical, genetic and pharmacologic) approach to define the role of Kinase Suppressor of Ras (KSR), a newly identified component of the EGFRIRaslRaflMAPK signaling pathway, in EGFR/Ras-mediated tumorigenesis and radio-response to IR. The hypotheses to be explored in this project are 1) oncogenic gf Ras, and hyper-activated wild-type (wt) Ras as a result of constitutively activated EGFR, signal oncogenesis via KSR; and 2) KSR is required for Ras-mediated radioresistance to IR. Specific aims to address these hypotheses are: 1) to investigate the role of KSR in EGFR/Ras tumorigenesis using transplanted human tumors, 2) to evaluate the role of KSR in EGFR/Ras-mediated responses to IR, and 3) to explore the potential of KSR as a molecular target for radiation therapy in various human tumors, specifically, the feasibility of using antisense oligodeoxynucleotide (AS-ODN) as a radiosensitizer. Results from these studies may offer new insights into the biological functions of KSR, which at this moment are largely unknown. Concomitantly, these investigations will help to identify new elements of Ras signaling which may have implications not only for the utility of KSR AS-ODNs in tumors with oncogenic ras mutations, but also in tumors with a highly activated wt Ras pathway as a result of overexpression of growth factors and/or their receptors. In summary, elucidation of KSR as an essential component of the EGFR/Ras-mediated tumorigenesis and cellular response to IR will potentially offer a highly tumor-specific molecular target for the development of diagnostic and therapeutic tools to treat a variety of human malignancies dependent on EGFR/Ras signaling.